U.S. stocks are in the red this afternoon after spending the entire morning in positive territory.

Futures got a lift before the opening bell from stronger-than-expected quarterly earnings results from Citi and a March retail sales report that also bested estimates, but the market hasn't been able to hold its previous gains.

The S&P 500 index is flat and the Nasdaq is 0.3% below Friday's closing levels. The iShares Nasdaq Biotechnology ETF, which has lost more than 20% of its value since late February, is down about 2% today.

A major focus for global markets to begin the week is the escalation of tensions between Ukraine and Russia.

Over the weekend, armed pro-Russian separatist forces occupied some government buildings and police stations in eastern Ukrainian cities, prompting an emergency meeting of the U.N. Security Council on Sunday night and threats of an anti-terrorism response from Ukrainian authorities. The separatists were given a deadline of 9 a.m. local time on Monday to surrender which expired this morning without a resolution to the standoff.

Russia's MICEX stock index closed 1.3% below Friday's closing levels, and the Russian ruble is the worst-performing major currency against the U.S. dollar. The dollar is also up sharply against many other emerging-market currencies as the asset class seems to have fallen out of favor, at least for today.

"It is very clear that Europe and the U.S. agree that Russia is fueling the current unrest, and in that sense at the very least a further wave of sanctions are very likely this week," says Derek Halpenny, European head of global markets research at Bank of Tokyo-Mitsubishi UFJ.

"German Finance Minister Schaeuble stated that there were 'many signs' that Russia was fanning the trouble. This in our mind remains by far the most likely source for turning financial market sentiment firmly into risk-off mode. The markets have been incredibly ambivalent to a situation that could turn suddenly."

The euro is trading near $1.38, down 0.5% against the U.S. dollar, following comments over the weekend from European Central Bank President Mario Draghi, who warned that a further rise in the euro could prompt additional monetary easing measures at IMF meetings in Washington, D.C.

"The strengthening of the exchange rate requires further monetary stimulus," he said.

"That’s an important dimension for our price stability."

European equity indices mostly closed higher, with the exception of Spain's IBEX 35, which finished about 0.2% lower.

The charts below show movements in various markets. Across the top, from left to right, are S&P 500 futures, the U.S. dollar-Japanese yen exchange rate, and the euro-U.S. dollar exchange rate. Across the bottom are gold futures, five-year U.S. Treasury note futures, and December 2015 eurodollar futures.